subtitles/en/63_data-processing-for-causal-language-modeling.srt

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(transition music)

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- In this video, we take a
look at the data processing

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necessary to train causal language models.

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Causal language modeling is the task

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of predicting the next token
based on the previous ones.

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Another term for causal language modeling

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is autoregressive modeling.

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In the example that you can see here,

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the next token could, for example,

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be NLP or it could be machine learning.

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A popular example of
causal language models

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is the GPT family of models.

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To train models such as GPT,

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we usually start with a
large corpus of text files.

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These files can be webpages
scraped from the internet

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such as the Common Crawl dataset

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or they can be Python files from GitHub,

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like the ones you can see here.

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As a first step, we need
to tokenize these files

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such that we can feed
them through the model.

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Here, we show the tokenized
texts as bars of various length,

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illustrating that they're
shorter and longer ones.

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This is very common
when working with text.

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However, transform models
have a limited context window

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and depending on the data source,

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it is possible that the tokenized texts

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are much longer than this window.

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In this case, we could
just truncate the sequences

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to the context length,

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but this would mean
that we lose everything

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after the first context window.

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Using the return overflowing token flag,

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we can use the tokenizer to create chunks

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with each one being the
size of the context length.

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Sometimes, it can still happen

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that the last chunk is too short

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if there aren't enough tokens to fill it.

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In this case, we can just remove it.

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With the return_length keyword,

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we also get the length of
each chunk from the tokenizer.

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This function shows all the steps

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necessary to prepare the dataset.

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First, we tokenize the dataset

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with the flags I just mentioned.

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Then, we go through each chunk

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and if it's length matches
the context length,

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we add it to the inputs we return.

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We can apply this function
to the whole dataset.

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In addition, we make sure

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that to use batches and
remove the existing columns.

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We need to remove the existing columns,

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because we can create
multiple samples per text,

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and the shapes in the dataset

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would not match anymore in that case.

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If the context length is of
similar lengths as the files,

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this approach doesn't
work so well anymore.

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In this example, both sample 1 and 2

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are shorter than the context size

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and will be discarded with
the previous approach.

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In this case, it is better
to first tokenize each sample

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without truncation

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and then concatenate the tokenized samples

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with an end of string
or EOS token in between.

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Finally, we can chunk this long sequence

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with the context length and we
don't lose too many sequences

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because they're too short anymore.

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So far, we have only talked

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about the inputs for
causal language modeling,

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but not the labels needed
for supervised training.

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When we do causal language modeling,

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we don't require any extra
labels for the input sequences

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as the input sequences
themselves are the labels.

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In this example, when we feed
the token trans to the model,

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the next token we wanted
to predict is formers.

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In the next step, we feed
trans and formers to the model

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and the label we wanted to predict is are.

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This pattern continues,
and as you can see,

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the input sequence is the label sequence

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just shifted by one.

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Since the model only makes
prediction after the first token,

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the first element of the input sequence,

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in this case, trans,
is not used as a label.

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Similarly, we don't have a label

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for the last token in the sequence

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since there is no token
after the sequence ends.

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Let's have a look at what we need to do

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to create the labels for causal
language modeling in code.

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If we want to calculate a loss on a batch,

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we can just pass the input_ids as labels

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and all the shifting is handled
in the model internally.

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So, you see, there's no matching involved

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in processing data for
causal language modeling,

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and it only requires a few simple steps.

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(transition music)